Publications

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Sandia National Laboratories partnered with a multi-disciplinary group of subject matter experts to evaluate a stratigraphic geothermal resource in Steptoe Valley, Nevada using both established and novel geophysical imaging techniques. The stratigraphic reservoir in northern Steptoe Valley was previously discovered during oil and gas exploration. Subsequent studies, such as the Nevada Play Fairway Analysis, included data which further highlighted potential resource targets in the basin. Geophysical surveys, complemented with refined geologic mapping and geochemical sampling, were deployed to further characterize the resource. The resulting 3D geologic interpretation, conceptual model refinements, and reservoir simulations suggest that a >100MWe power-capable reservoir is likely economically accessible using conventional well placement and stimulation techniques in the Paleozoic carbonates of the deep/central basin of northern Steptoe Valley. Additional geophysical characterization and exploration drilling efforts are recommended to calibrate interpretation and determine where/how to potentially develop the northern Steptoe resource. The geophysical tools, interpretations, lessons learned, and public data generated by this study establish an exploration methodology to inform decisions for characterization and development of northern Steptoe Valley and other stratigraphic geothermal reservoirs in the western U.S.

Fluid flow through fractured media is typically governed by the distribution of fracture apertures, which are in turn governed by stress. Consequently, understanding subsurface stress is critical for understanding and predicting subsurface fluid flow. Although laboratory-scale studies have established a sensitive relationship between effective stress and bulk electrical conductivity in crystalline rock, that relationship has not been extensively leveraged to monitor stress evolution at the field scale using electrical or electromagnetic geophysical monitoring approaches. In this paper we demonstrate the use time-lapse 3-dimensional (4D) electrical resistivity tomography to image perturbations in the stress field generated by pressurized borehole packers deployed during shear-stimulation attempts in a 1.25 km deep metamorphic crystalline rock formation.

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Blind geothermal systems are believed to be common in the Basin and Range province and represent an underutilized source of renewable green energy. Their discovery has historically been by chance but more methodological strategies for exploration of these resources are being developed. One characteristic of blind systems is that they are often overlain by near-surface zones of low-resistivity caused by alteration of the overlying sediments to swelling clays. These zones can be imaged by resistivity-based geophysical techniques to facilitate their discovery and characterization. Here we present a side-by-side comparison of resistivity models produced from helicopter transient electromagnetic (HTEM) and ground-based broadband magnetotelluric (MT) surveys over a previously discovered blind geothermal system with measured shallow temperatures of ~100°C in East Hawthorne, NV. The HTEM and MT data were collected as part of the BRIDGE project, an initiative for improving methodologies for discovering blind geothermal systems. HTEM data were collected and modelled along profiles, and the results suggest the method can resolve the resistivity structure 300 - 500 m deep. A 61-station MT survey was collected on an irregular grid with ~800 m station spacing and modelled in 3D on a rotated mesh aligned with HTEM flight directions. Resistivity models are compared with results from potential fields datasets, shallow temperature surveys, and available temperature gradient data in the area of interest. We find that the superior resolution of the HTEM can reveal near-surface details often missed by MT. However, MT is sensitive to several km deep, can resolve 3D structures, and is thus better suited for single-prospect characterization. We conclude that HTEM is a more practical subregional prospecting tool than is MT, because it is highly scalable and can rapidly discover shallow zones of low resistivity that may indicate the presence of a blind geothermal system. Other factors such as land access and ground disturbance considerations may also be decisive in choosing the best method for a particular prospect. Resistivity methods in general cannot fully characterize the structural setting of a geothermal system, and so we used potential fields and other datasets to guide the creation of a diagrammatic structural model at East Hawthorne.

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This report documents the development of the Blue Canyon Dome (BCD) testbed, including test site selection, development, instrumentation, and logistical considerations. The BCD testbed was designed for small-scale explosive tests (~5 kg TNT equivalence maximum) for the purpose of comparing diagnostic signals from different types of explosives, the assumption being that different chemical explosives would generate different signatures on geophysical and other monitoring tools. The BCD testbed is located at the Energetic Materials Research and Testing Center near Socorro, New Mexico. Instrumentation includes an electrical resistivity tomography array, geophones, distributed acoustic sensing, gas samplers, distributed temperature sensing, pressure transducers, and high-speed cameras. This SAND report is a reference for BCD testbed development that can be cited in future publications.

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The Basin & Range Investigations for Developing Geothermal Energy (BRIDGE) Project kicked off in the Autumn of 2021. The Department of Energy Geothermal Technologies Office (GTO) funded BRIDGE as part of a broader GTO initiative to advance the identification and development of hidden, or “blind”, geothermal energy resources in the Basin and Range Province (Basin & Range) of the western USA. The BRIDGE Team is a collaboration being led by Sandia National Laboratories (Sandia) with partners from Geologica Geothermal Group, the US Navy Geothermal Program Office, and others that will contribute to various stages of the project. The focus of this project is on Western Nevada with areas of interest, identified chiefly from the prior Nevada Play Fairway Analysis (PFA) study, located primarily in Churchill and Mineral Counties including lands managed by the Department of Defense (DOD). The first stage of BRIDGE is focused on reconnaissance of PFA targets that are suspected or known to be associated with hidden geothermal resources on DOD and surrounding lands. Helicopter-borne transient electromagnetism (HTEM) surveying is being used in a novel conceptual approach for optimizing shallow and deep well targeting in Basin & Range geothermal exploration. This reconnaissance phase is part of the overall BRIDGE workflow: 1. Assess the pre-survey likelihood of geothermal systems in the study area based on PFA reviews and a reanalysis of existing information to constrain subsurface temperature, structure, hydrology, and thermal manifestations. 2. Design and execute HTEM resistivity surveying to image the depth to the low resistivity and low permeability clay cap, within which a thermally conductive (linear) temperature gradient could be targeted for drilling, and potentially image the underlying higher resistivity associated with shallow aquifers hosting outflows from deeper geothermal systems. 3. Drill temperature gradient (TG) wells that penetrate a thick enough section of the clay cap detected by HTEM surveying to provide a linear thermal gradient that could be reliably extrapolated to the base of the cap. 4. In areas where the TG wells detected a prospective temperature gradient but where the HTEM survey did not penetrate to the base of the cap, conduct surface magnetotelluric (MT) resistivity surveys to image the base of the cap to identify the depth to which the linear TG well gradient could be reliably extrapolated. 5. On the most prospective target(s), drill at least one testable slim hole well to discover the resource associated with the interpreted geothermal reservoir upflow source. The first stage of the project and the second stage HTEM survey have been completed. Preliminary results are being analyzed with respect to potential TG targets and plans for followup surveys, geophysical joint inversion, conceptual model development, and interpretation.

The DOE GeoVision study identified that Enhanced Geothermal Systems (EGS) resources have the potential to provide a significant contribution toward achieving the goal of converting the U.S. electricity system to 100% clean energy over the next few decades. To further the implementation of commercial EGS development, DOE's Geothermal Technologies Office (GTO) initiated the Wells of Opportunity (WOO) Amplify program, where unproductive wells in selected geothermal fields are to be stimulated using EGS technologies, resulting in increased power production from these resources. As part of the WOO-Amplify project, GTO assembled the Amplify Monitoring Team (AMT), whose role is to provide in-field and near-field seismic monitoring design, deployment and data analysis for stimulations under the WOO-Amplify initiative. This team, consisting of scientists and engineers from Lawrence Berkeley National Laboratory (LBNL), Sandia National Laboratories (SNL), and the US Geological Survey (USGS), is working with WOO-Amplify EGS Operators in Nevada to develop and deploy optimized seismic monitoring systems at four geothermal fields where WOO-Amplify well stimulations are planned: Don A. Campbell, Tungsten Mountain and Jersey Valley operated by Ormat Technologies, and Patua operated by Cyrq Patua Acquisition Company LLC. Using geologic and geophysical field data provided by the WOO-Amplify teams, the focus of the AMT is to develop advanced simulation and modeling techniques, design targeted seismic monitoring arrays, develop innovative and cost-effective methodologies for drilling seismic monitoring boreholes, deploy effective seismic instrumentation, and facilitate the use of microseismic data to monitor well stimulation and flow within the geothermal reservoir. Realtime seismic data from the four WOO-Amplify sites will be streamed to a publicly accessible Amplify Monitoring website. AMT's advanced simulations and template matching techniques applied during pre-stimulation phases can help improve understanding of potential seismic hazard and inform the Operator's Induced Seismicity Mitigation Protocol (ISMP). Over the next two years, AMT will be drilling, instrumenting, and recording seismic data at the WOO-Amplify field sites, telemetering the seismic waveform data to AMT's central processing system and providing the processed location data to the WOO Amplify Operator teams. These data and monitoring systems will be critical for effective monitoring of the effects of planned well stimulation and extended flow tests during the next stage of the WOO-Amplify project.

Sedimentary-hosted geothermal energy systems are permeable structural, structural-stratigraphic, and/or stratigraphic horizons with sufficient temperature for direct use and/or electricity generation. Sedimentary-hosted (i.e., stratigraphic) geothermal reservoirs may be present in multiple locations across the central and eastern Great Basin of the USA, thereby constituting a potentially large base of untapped, economically accessible energy resources. Sandia National Laboratories has partnered with a multi-disciplinary group of collaborators to evaluate a stratigraphic system in Steptoe Valley, Nevada using both established and novel geophysical imaging techniques. The goal of this study is to inform an optimized strategy for subsequent exploration and development of this and analogous resources. Building from prior Nevada Play Fairway Analysis (PFA), this team is primarily 1) collecting additional geophysical data, 2) employing novel joint geophysical inversion/modeling techniques to update existing 3D geologic models, and 3) integrating the geophysical results to produce a working, geologically constrained thermo-hydrological reservoir model. Prior PFA work highlights Steptoe Valley as a favorable resource basin that likely has both sedimentary and hydrothermal characteristics. However, there remains significant uncertainty on the nature and architecture of the resource(s) at depth, which increases the risk in exploratory drilling. Newly acquired gravity, magnetic, magnetotelluric, and controlled-source electromagnetic data, in conjunction with new and preceding geoscientific measurements and observations, are being integrated and evaluated in this study for efficacy in understanding stratigraphic geothermal resources and mitigating exploration risk. Furthermore, the influence of hydrothermal activity on sedimentary-hosted reservoirs in favorable structural settings (i.e., whether fault-controlled systems may locally enhance temperature and permeability in some deep stratigraphic reservoirs) will also be evaluated. This paper provides details and current updates on the course of this study in-progress.

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The final version of the above article was posted prematurely on 16 July 2021, owing to a technical error. The final, corrected version of record will be made fully available at a later date.

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Enhanced Geothermal Systems could provide a substantial contribution to the global energy demand if their implementation could overcome inherent challenges. Examples are insufficient created permeability, early thermal breakthrough, and unacceptable induced seismicity. Here we report on the seismic response of a mesoscale hydraulic fracturing experiment performed at 1.5‐km depth at the Sanford Underground Research Facility. We have measured the seismic activity by utilizing a 100‐kHz, continuous seismic monitoring system deployed in six 60‐m length monitoring boreholes surrounding the experimental domain in 3‐D. The achieved location uncertainty was on the order of 1 m and limited by the signal‐to‐noise ratio of detected events. These uncertainties were corroborated by detections of fracture intersections at the monitoring boreholes. Three intervals of the dedicated injection borehole were hydraulically stimulated by water injection at pressures up to 33 MPa and flow rates up to 5 L/min. We located 1,933 seismic events during several injection periods. The recorded seismicity delineates a complex fracture network comprised of multistrand hydraulic fractures and shear‐reactivated, preexisting planes of weakness that grew unilaterally from the point of initiation. We find that heterogeneity of stress dictates the seismic outcome of hydraulic stimulations, even when relying on theoretically well‐behaved hydraulic fractures. Once hydraulic fractures intersected boreholes, the boreholes acted as a pressure relief and fracture propagation ceased. In order to create an efficient subsurface heat exchanger, production boreholes should not be drilled before the end of hydraulic stimulations.

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Explosions detonated in geologic media damage it in various ways via processes that include vaporization, fracturing, crushing of interstitial pores, etc. Seismic waves interact with the altered media in ways that could be important to the discrimination, characterization, and location of the explosions. As part of the Source Physics Experiment, we acquired multiple pre- and post-explosion near-field seismic datasets and analyzed changes to seismic P-wave velocity. Our results indicate that the first explosion detonated in an intact media can cause fracturing and, consequently, a decrease in P-wave velocity. After the first explosion, subsequent detonations in the pre-damaged media have limited discernible effects. We hypothesize this is due to the stress-relief provided by a now pre-existing network of fractures into which gasses produced by the explosion migrate. We also see an overall increase in velocity of the damaged region over time, either due to a slow healing process or closing of the fractures by subsequent explosions.